1. Field of the Invention
The present invention relates to a photo-sensing device used as a photo-sensor of a light communication system.
2. Related Background Art
FIGS. 1A and 1B respectively show a structure of a prior art photo-sensing device. FIG. 1A shows a top view and FIG. 1B shows an X--X sectional view. As shown, in the prior art photo-sensing device, an electrode 8 having a center opening through which light is directed is formed on an underside of a semiconductor substrate 1 of a first conductivity type which is transparent to the light to be detected, and an anti-reflection film 9 is formed in the opening. A semiconductive crystal layer 2 of the first conductivity type for absorbing the incident light is formed on the surface of the semiconductor substrate 1. The semiconductive crystal layer 2 is a lamination of a buffer layer 2a, a photo-sensing layer 2b and a cap layer 2c in sequence. Impurities are selectively diffused into the semiconductive crystal layer 2 to form a first region 3 of a second conductivity type. This is a pin photo-diode structure where the semiconductor substrate 1 is an n layer (or a p layer), the semiconductive crystal layer 2 is an i layer and the first region 3 is a p layer (or an n layer), and a photo-sensing region 10 is formed in the i layer. An electrode 5 for taking out a photo-current is formed on the first region 3, the surface of the semiconductive crystal layer 2 around the electrode 5 is covered with a protection film (i.e. passivation film) 7.
When a reverse bias is applied to the semiconductor device thus constructed, a depletion layer is created in a pn junction area in the semiconductive crystal layer 2. Thus, an electric field is developed in the depletion layer and electrons and holes generated by a light applied to the photo-sensing region 10 are directed to the semiconductor substrate 1 and the first region 3 of different conductivity types, respectively, and accelerated thereby. In this manner, a photo-current is taken out and a light signal is detected.
A similar structure of a photo-sensing device to that described above is disclosed in U.S. Pat. No. 6,093,174 (issued on '84,05.11).
In the structure shown in FIGS. 1A and 1B, when the light is applied to the photo-sensing region 10, light generating carriers are captured by the depletion layer and a good response characteristic is offered. However, when the light is directed to the outside of the region 10, the generated carriers reach the pn junction while they are diffused by a density gradient and are taken out as a photo-current. As a result, the response characteristic is adversely affected. FIG. 2A shows a response characteristic of the photo-sensing device. Since the movement of the carriers by the diffusion is slow, a response waveform to a light pulse includes a tail at the end as shown in FIG. 2A.
When such a photo-sensing device is used for light communication, a light emitted from an optical fiber is focused so that it is directed to the photo-sensing region 10. However, when a portion of light leaks out of the photo-sensing region 10, it leads to the reduction of the response speed of the photo-sensing device by the reason described above. In a high speed photo-sensing device, the area of the photo-sensing region 10 is reduced for reduction of a junction capacitance. As a result, a ratio of light directed to the outside of the photo-sensing region 10 increases and a diffused component which has a low response speed increases. This leads to the degradation of the response speed.
A rear-entry type structure as shown is suitable for use as a high response speed device because of a small junction area. However, when it is coupled to a single mode fiber (core diameter 10 .mu.m), a portion of an incident light leaks to the outside of the photo-sensing region 10 due to a deviation of an optical axis or an aberration of a lens. The length of diffusion of carriers is in the order of several tens .mu.m (approximately 40 .mu.m in the n-type indium-gallium-arsenide layer with the indium-phosphide cap layer), and the carriers generated at a distant point from the pn junction also contribute to the photo-current by the diffusion. This also leads to the reduction of the response speed.